1. Field of the Invention
The present invention relates to an externally pressurized gas bearing assembly for an externally pressurized gas bearing spindle device such as those utilized in a precision machining tool, a precision testing apparatus or the like.
2. Description of the Prior Art
The externally pressurized gas bearing spindle device includes a main shaft rotatably supported by the externally pressurized gas bearing assembly in a non-contact fashion. The externally pressurized gas bearing spindle device can give rise to a highly precise control of the rotational speed and is generally utilized in conjunction with a work spindle such as those utilized in a precision machining tool or a precision testing apparatus, or a tool spindle. Most of the prior art externally pressurized gas bearing spindle devices are made up of a number of housing assembly components. In consideration of the durability (the anti-seizing property) against the possibility of the bearing assembly contacting the main shaft a housing assembly component is constructed by press-fitting a bearing sleeve, made of a material suitable for the gas bearing assembly, into a housing made of a construction material.
One example of the prior art externally pressurized gas bearing spindle devices is shown in FIG. 4. The housing assembly component is constructed by press-fitting a bearing sleeve 2 anally into each of the opposite ends of a housing 1. A compressed air to the externally pressurized gas bearing assembly is supplied from an air supply port 11 in the housing 1 to a circumferential row of a plurality of journal bearing air supply ports 21, formed in a circumferential row in each of the bearing sleeves 2, and also to a plurality of thrust bearing air supply ports 22, similarly formed in a cir-circumferential circumferential row in each of the bearing sleeves 2, through passages 12 by way of annular grooves 13 and 14.
The externally pressurized gas bearing spindle device shown in FIG. 4 has a structure wherein respective contact surfaces of the housing 1 and each bearing sleeve 2 are brought into tight contact with each other to provide a seal against the flow of the compressed air. The contact surface B of the journal bearing portion can tightly contact the mating contact surface of the bearing sleeve 2, in view of the press-fit between the housing 1 and the bearing sleeve 2, to provide an excellent seal. However, a disadvantage of the prior art device is that it is difficult to achieve a tight contact between the housing 1 and the bearing sleeve 2 at a contact surface A of the thrust bearing portion. The reason therefor is because a force required to achieve a tight contact therebetween is not achieved; furthermore, a tight contact cannot be attained particularly where the diameter is so large that the contact surface cannot be formed precisely enough. For this reason, a sufficient seal will not be attained at a portion in the vicinity of the annular groove 14. Once the compressed air leaks because of the insufficient sealing, the amount of the compressed air consumed by the bearing assembly will increase, accompanied by reduction in ability of the bearing assembly to support the load, and this is detrimental to the performance of the bearing assembly.
Also, in the externally pressurized gas bearing assembly shown in FIG. 4, when the compressed air is supplied to the annular groove 14, an axially acting force (Compressed Air Pressure.times.Surface Area of a Portion of Contact Surface A Occupied by Groove 14) will be generated by the pressure of the compressed air at the contact surface A of the thrust bearing portion of the bearing sleeve 2. This axially acting force deforms the thrust bearing portion of the bearing sleeve 2 and also, pushes the bearing sleeve 2 out of the housing 1. Deformation of the thrust bearing portion of the bearing sleeve 2 will render a trust bearing gap to be uneven to such an extent as to adversely affect the performance of the bearing assembly and also reduce the sealing effect at a portion of the annular groove 14.
In addition, in the event that the compressed air leaks from the entire contact surface A of the thrust bearing portion an axially acting force of a magnitude (Compressed Air Pressure.times.Surface Area of Entire Contact Surface A) larger than the previously discussed anally acting force will act on the contact surface A of the bearing sleeve 2 due to the pressure of the leaking compressed air.
Many of the prior art devices use copper-containing alloy, for example, a gun metal, lead-containing bronze or beryllium-containing copper, for the bearing sleeves. Recently, the use of graphite as a material has increased due to its anti-seizing property. When a material having a low elasticity and a low strength, such as graphite, is used for the bearing sleeves, the influence brought about by the force with which the previously discussed compressed air acts on the bearing sleeve will be large. In the event that the thrust bearing portion of the bearing sleeve is deformed by the effect of the axially acting force brought about by the compressed air on the bearing sleeve, the magnitude of deformation will be considerably more than if metal is used for the bearing sleeve; thus, bearing performance will be affected adversely.
If a graphite sleeve is employed in the externally pressurized gas bearing assembly of the structure shown in FIG. 4, in view of the fact that the graphite has a relatively low strength and a relatively low elasticity, the press-fitting of the bearing sleeve 2 into the housing 1 will not result in an increase in contact pressure at the contact surface A of the journal bearing portion, as is exhibited with the use of the metal. In other words, given an allowance for the interference fit, the use of the graphite having a relatively low elasticity results in a lower contact pressure than that exhibited when metal is used. Also, if the interference for the interference fit is increased in order to increase the contact pressure, the use of graphite having a relatively low strength will result in breakage at a smaller interference for the interference fit than that as afforded by the metal. In view of this, due to the graphite having a low coefficient of friction, the retaining force of the bearing sleeve 2 brought about by the interference fit is low as compared with that brought about by the metal. Consequently, there is a possibility that by the action of the axially acting force brought about by the compressed air on the bearing sleeve, the bearing sleeve may separate from the housing.
Thus, when the graphite is used for the bearing sleeves 2 more consideration than that required where the metal is used is required in view of the problems discussed hereinabove. However, where the bearing has a large diameter, sealing of the compressed air at the contact surface A of the thrust bearing portion is difficult to achieve, thus adverse influence brought about by the compressed air is considerable. Therefore, it is difficult to design a spindle device providing proper functionality within the internal structure of the housing assembly component as is found in the prior art externally pressurized gas bearing assembly discussed above.